Circuit for blocking a semiconductor switching device on overcurrent

ABSTRACT

A conventional circuit for blocking a semiconductor switching device ( 7 ) on overcurrent, the semiconductor switching device ( 7 ) having at least one continuously driven semiconductor switch ( 9 ), comprises a diver circuit ( 11 ), which has a driver stage ( 12 ) for each semiconductor switch ( 9 ), a control pulse generator ( 10 ) for producing control pulses (P 1  to P 6 ), which are fed in operation to a control input of the semiconductor switching device ( 7 ) via the driver circuit ( 11 ), and a monitoring device ( 34 ), which measures the current (I) flowing through the semiconductor switching device ( 7 ) and, when an overcurrent occurs, generates a fault signal (E), which initiates blocking of the semiconductor switching device ( 7 ). When the semiconductor switching device is being blocked, a high overvoltage can occur therein, possibly leading to destruction of the semiconductor switching device. In order to reduce, with little outlay, the overvoltage in the semiconductor switching device during blocking thereof, provision is made such that the operating voltage (U) of the driver circuit ( 11 ) is arranged to be switched over briefly by the fault signal to a lower, interim value corresponding to a lower current (I) through the semiconductor switching device ( 7 ) and then, within the maximum permissible duration for loading the semiconductor switching device ( 7 ) with an overcurrent, to be switched off completely.

BACKGROUND OF THE INVENTION

The invention relates to a circuit for blocking a semiconductorswitching device on overcurrent, the semiconductor switching devicehaving at least one continuously driven semiconductor switch, whichcircuit comprises a driver circuit having a driver stage for eachsemiconductor switch, a control pulse generator for producing controlpulses, which are fed in operation to a control input of thesemiconductor switching device via the driver circuit, and a monitoringdevice, which measures the current flowing through the semiconductorswitching device and which, when an overcurrent occurs, generates afault signal, which initiates blocking of the semiconductor switchingdevice.

The semiconductor switching device is generally an inverter havingseveral power switching transistors in the form of semiconductorswitches.

In a known circuit of that type (EP 0 521 260 B1), free-wheeling diodesare connected anti-parallel to each semiconductor switch in order toavoid, at the semiconductor switches, overvoltages that are caused byinductive resistors, such as choke coils, inductive loads or leadinductances, in the circuitry of the semiconductor switches when asemiconductor switch is switched off (blocked) in normal operation. Whenan overcurrent, for example a short-circuit current, flows through thesemiconductor switches, it is, however, possible for even higherovervoltages to occur. The known circuit should reduce thoseovervoltages by blocking one of the series-connected semiconductorswitches simultaneously carrying an overcurrent, without increasing theamount of circuitry involved by using capacitors. Notwithstanding,free-wheeling diodes are still required. Even when those are provided,when a semiconductor switch carrying a very high overcurrent, such as ashort-circuit current, is being blocked, in the circuit of whichsemiconductor switch there is a high inductive reactance, a very highovervoltage can still occur at the blocked semiconductor switch.

SUMMARY OF THE INVENTION

The invention is based on the problem of providing a circuit of the kindmentioned at the beginning that allows, with little outlay, a furtherreduction in an overvoltage at the semiconductor switching device whenthat is being blocked because of an overcurrent.

According to the invention, that is achieved by means of the fact thatthe operating voltage of the driver circuit is arranged to be switchedover briefly by the fault signal to a lower, interim value correspondingto a lower current through the semiconductor switching device and then,within the maximum permissible duration for loading the semiconductorswitching device with an overcurrent, to be switched off completely.

In this solution, therefore, the overcurrent is reduced to zero instages. For each switching-off stage, the amount by which the currentflowing through the semiconductor switching device decreases is,therefore, also smaller. Consequently, the rate of change (di/dt) of thecurrent is correspondingly lower for each switching-off stage, as is,therefore, the voltage induced in the inductive reactance in the circuitof the semiconductor switching device by the change in the current(Ldi/dt). Because the induced voltage is added to the operating voltageof the semiconductor switching device when the semiconductor switchingdevice is being blocked, the total overvoltage at the semiconductorswitching device when the blocking occurs is also lower. Thesemiconductor switching device is, therefore, not unduly loaded and doesnot require additional circuitry to reduce overvoltage when blockingoccurs.

Provision is preferably made such that, for several semiconductorswitches jointly supplied from one operating voltage source, the currentflowing through the semiconductor switches is measured in a supply linecommon to all th e semiconductor switches by the monitoring device, asingle measuring device in the monitoring device being sufficient forall the semiconductor switches.

Provision can then be made such that, for several semiconductorswitches, the driver stages thereof are all supplied from a commonoperating voltage source, which is galvanically isolated from the driverstages and which, as a function of the fault signal, is arranged to beswitched over to the interim value and switched off. In thatarrangement, there is no need, when an overcurrent occurs in asemiconductor switch, to determine which semiconductor switch isaffected. There is, accordingly, less outlay on resources in themonitoring device.

An advantageous practical form of the circuit can consist in that theoperating voltage source of the driver circuit is a direct-currentvoltage source, which is connected, via a chopper controlled by a pulsedswitching signal and a transformer having a secondary winding for eachdriver stage and via a rectifying circuit connected to the secondarywinding, to a (respective) driver stage and the switching signal thatcontrols the chopper is frequency- or pulse-length-modulated as afunction of the fault signal. In that arrangement, the reduction in theoperating voltage of the driver circuit when an overcurrent occurs isachieved by conversion of the operating voltage into a pulsed voltageand subsequent frequency- or pulse-length-modulation of the pulsedvoltage.

The control pulses of the control pulse generator can be fed to acontrol input of the driver circuit in a customary manner.

Provision is preferably made such that the control pulses and theoperating voltage for each driver stage are transmitted by means of ahigh-frequency carrier signal of an oscillator that is common to all thedriver stages, via the same galvanic isolation stage. In thatarrangement, galvanic isolation between the switching device, which isoptionally operated with high voltage, and the low-voltage-operatedswitching circuits controlling the driver stage(s) thereof is possiblewith little outlay on isolation stages.

A simple practical form can consist in combining the control pulses ofthe control pulse generator (when using frequency- orpulse-length-modulation of a driver circuit direct-current operatingvoltage converted into a pulsed intermediate circuit voltage by means ofthe chopper) with the switching signal controlling the chopper by meansof an AND gate, in order to transmit the control pulses and theoperating voltage to the driver stages with galvanic isolation.

A further alternative arrangement of the operating voltage source of thedriver circuit can consist in that it has an output for a high, normalvalue and an output for the low, interim value, one of which outputs canbe selected for the supply of the operating voltage as a function of thefault signal.

In that arrangement, the outputs can be connected by an OR gate.

Provision can then be made such that one output is connected, via adiode, to one end of the switching path of a controllable switch, theother output is connected, via a diode, to the other end of theswitching path and to the driver circuit, and the switch is arranged tobe switched as a function of the fault signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its developments are described below in greater detailwith reference to drawings of preferred embodiments.

FIG. 1 shows a first embodiment of a circuit according to the invention,used with an inverter;

FIGS. 2a)-c) shows graphs illustrating the basic principle of theinvention;

FIG. 3 shows a modification of the circuit according to FIG. 1; and

FIG. 4 shows a further modification of the circuit according to FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to FIG. 1, an operating voltage source 1, here adirect-current voltage source, comprising a three-phase bridge rectifier2, a smoothing inductor 3 and a smoothing capacitor 4, is connected, viasupply lines 5 and 6, to a semiconductor switching device 7 in the formof an inverter for three-phase alternating current for the supply of analternating-current load 8, here a three-phase alternating-currentmotor. The semiconductor switching device 7 comprises three seriescircuits each comprising two continuously driven semiconductor switches9 connected parallel to the supply lines 5, 6, the interconnectionpoints of the semiconductor switches 9 being connected to a respectivephase of the alternating-current load 8. The semiconductor switches 9are switching transistors, especially field-effect transistors,preferably IGBT's (INTEGRAL GATE BIPOLAR TRANSISTORS), that is to saybipolar transistors having an integral gate, for high power levels.

The control connections of the semiconductor switches 9 are fed by acontrol pulse generator 10 with control pulses P₁ to P₆, which arephase-shifted in accordance with the desired switching sequence and havethe desired operating frequency of the alternating-current load 8, via adriver circuit 11, which has, for each semiconductor switch 9, arespective driver stage 12 connected on the output side to the controlconnection of one of the semiconductor switches 9. The control inputs ofthe driver stages 12 are each connected to a control pulse output of thecontrol pulse generator 10, as illustrated for one driver stage 12.

The driver circuit 11 receives a direct-current operating voltage from acurrent supply device, which is constructed as follows: the primarywinding 13 of a transformer 14 and a switching transistor 15 areconnected to the operating voltage source 1 in series between the supplylines 5 and 6. The ends of a secondary winding 16 of the transformer 14,which is provided with further secondary windings 17 and 18, areconnected to the series circuit comprising a diode 19 for the purpose ofrectification and a smoothing capacitor 20. Between a centre tap of thesecondary winding 16 and one end of the secondary winding 16 there isconnected a further series circuit comprising a diode 21 for the purposeof rectification and a smoothing capacitor 22. The smoothing capacitor20 is connected to the series circuit comprising a switching transistor23 and a chopper in the form of an inverter formed by fourbridge-connected switching transistors 24, 24′, 25, 25′ andfree-wheeling diodes connected parallel to the switching transistors 24,24′, 25, 25′; the smoothing capacitor 22 is connected to the inverteronly. In the null path of the bridge there is connected the seriescircuit comprising a capacitor 26 and a primary winding 27 of atransformer 28. The transformer 28 has a secondary winding 29 for eachdriver stage 12, only two of which secondary windings 29 are shown. Eachsecondary winding 29 is connected to the series circuit comprising adiode 30 for the purpose of rectification and a smoothing capacitor 31,only one of those series circuits being shown, in order to simplify therepresentation. Each capacitor 31 is connected to the current supplyconnections of a respective driver stage 12.

A voltage controller 32, which detects the output voltage of the voltagesource 1 and compares it with a set value, causes, via an oscillator 33having a controllable frequency determined as a function of the controlerror detected by the voltage controller 32, the switching transistor 15(likewise a field-effect transistor) to be switched on and offperiodically at the frequency of the oscillator 33. The switchingfrequency of the switching transistor 15 determines the inductivereactance of the primary winding 13 of the transformer 14 and, as aresult, the voltage drop at the primary winding 13, on which voltagedrop the output voltage at the secondary winding 16 in turn depends. Thevoltage controller 32 therefore ensures that the output voltage at thesecondary winding 16 is largely constant irrespective of fluctuations inthe output voltage of the operating voltage source 1. Consequently, thedirect-current voltages occurring at the smoothing capacitors 20 and 22are largely independent of fluctuations in the output voltage of theoperating voltage source 1. The secondary windings 17 and 18 are used tosupply current to components in the circuit, for example the voltagecontroller 32 and the oscillator 33.

A monitoring device 34 comprises a current sensor 35, which measures ata central location the current flowing through the semiconductorswitching device 7 and all the semiconductor switches 9 in the supplyline 6, and a control device 36, the operating voltage of which is takenfrom the smoothing capacitor 20, which control device 36 compares thecurrent measured by the current sensor 35 with a reference value and,when there is an overcurrent, such as a short-circuit current, sends afault signal E to the control connection of the switching transistor 23via a line 38 and, after a delay, sends the fault signal to anoscillator 37 via a line 39. The oscillator 37 generates, at twooutputs, inversely related pulses and sends those pulses, on the onehand, to the control connections of the switching transistors 24, 25′and, on the other hand, to the control connections of the switchingtransistors 24′, 25.

The mode of operation of the arrangement illustrated in FIG. 1 isdescribed below in greater detail, with reference also being made toFIG. 2. FIG. 2a shows the waveform of the operating voltage U in thedriver stages 12; FIG. 2b shows the waveform of the current I flowingthrough the semiconductor switching device 7; and FIG. 2c shows thewaveform of the voltage U_(s) at a semiconductor switch 9.

As long as the monitoring device 34 does not detect an overcurrent, nofault signal E is fed to the switching transistor 23, with the resultthat it remains driven and the direct-current voltage at the smoothingcapacitor 20 is present at the series circuits comprising the switchingtransistors 24, 25 and 24′, 25′, which are likewise field-effecttransistors. Until an overcurrent is detected, the oscillator 37 islikewise continuously in operation and switches the series circuitscomprising the switching transistors 24, 25 and 24′, 25′ in push-pullmode, that is to say alternately, via its output lines. The square-wavealternating-current voltage available at that time in the null path ofthe bridge comprising the switching transistors 24, 25, 24′, 25′, isdivided up in accordance with the frequency of the oscillator 37 and thesquare-wave alternating-current voltage by means of the series circuitcomprising the capacitor 26 and primary winding 27, which acts as avoltage divider, and induced, according to the transformation ratio ofthe transformer 28, in the secondary windings 29 thereof. The inducedvoltage is rectified by means of the diode 30 and smoothed by means ofthe capacitor 31 and applied to the relevant driver stage 12 in the formof operating voltage U. As a result, the driver stage 12 continues tooperate and transmits the pulses, which are fed to it by the controlpulse generator 10, to the control connection of the relevantsemiconductor switch 9.

At time-point t₁ according to FIG. 2b, an overcurrent occurs, which isdetected by the monitoring device 34 with a slight delay at time-pointt₂ (because of its response delay). The control device 36 generates thefault signal E, which blocks the switching transistor 23. While theoperating voltage U of the driver stages 12 maintained its high, nominalvalue from time-point t₀ until time-point t₂, at time-point t₂ thevoltage at the inverter formed by the switching transistors 24, 25, 24′,25′ switches over to the lower, direct-current voltage at the smoothingcapacitor 22. As a result, at time-point t₂, the operating voltage U atthe driver stages 12 also drops, as shown in FIG. 2a, as does, at thesame time, the output current of the driver stages 12, so that thecurrent I flowing through the semiconductor switching device 7 isreduced as a result of partial blockage at the semiconductor switches 9,as shown in FIG. 2b. Because the secondary winding 16 of the transformer14 is tapped approximately in the centre, just half the voltageavailable at the smoothing capacitor 20 is available also at thesmoothing capacitor 22. Consequently, the operating voltage U at thedriver stages 12 drops to approximately half when the overcurrent isdetected at time-point t₂. The current I is accordingly reduced to halfat time-point t₂. After a delay, at time-point t₃, the control device 36sends the fault signal to the oscillator 37 via the line 39 in the formof a blocking signal, with the result that operation of the oscillator37 is interrupted and, therefore, the switching transistors 24, 25 and24′, 25′ are no longer switched alternately on and off. There is,therefore, no longer any voltage at the primary winding 27, with theresult that the transformer 28 transmits no voltage and, therefore, theoperating voltage U is likewise switched off at time-point t₃ accordingto FIG. 2a. Consequently, the current I at time-point t₃ according toFIG. 2b is also interrupted. The time from the occurrence of theovercurrent at time-point t₁ to switching off of the semiconductorswitching device 7 at time-point t₃ has been given a value such that itis shorter than the maximum permissible duration for loading thesemiconductor switching device 7 with an overcurrent. When the current Iis switched off at time-point t₃ there occurs only a slight overvoltageU_(so) (FIG. 2c) at the semiconductor switch 9 carrying the overcurrent.In contrast, if the semiconductor switching device 7 were to becompletely switched off at time-point t₂ when an overcurrent isdetected, a very much higher overvoltage would occur at the relevantsemiconductor switch 9, as shown by the broken line in FIG. 2c. That isexplained by the fact that, when the semiconductor switch 9 carrying theovercurrent is being blocked in stages according to the invention, therate of change di/dt of the current I at time-points t₂ and t₃ is onlyabout half that that would arise from full blocking at time-point t₂and, as a result, the voltage induced in an inductive reactance, forexample the smoothing coil 3 and/or a coil in the alternating-currentload 8 and/or the inductance of a lead, which induced voltage is addedto the normal operating direct-current voltage of the semiconductorswitch 9 in question when that semiconductor switch 9 is being blocked,is reduced according to the relation Ldi/dt because di drops, L beingthe inductance of the inductive reactance. There is therefore no needfor additional circuitry in the semiconductor switches 9 to reduce suchan overvoltage when one of the semiconductor switches 9 is beingblocked.

FIG. 3 shows a portion of the arrangement according to FIG. 1 that hasbeen modified with respect to the arrangement according to FIG. 1.Accordingly, compared with the arrangement according to FIG. 1, thecentre tap of the secondary winding 16 of the transformer 14, the diode21, the capacitor 22 and the switching transistors 23, 24′, 25, 25′ areomitted. Instead of those switching transistors, only the switchingtransistor 24 is still connected in series with the capacitor 26 and theprimary winding 27. Furthermore, instead of the oscillator 37, acontrollable oscillator 40 is provided, the single output of which isconnected to the control input of the switching transistor 24 and which,when the fault signal occurs at time-point t₂ according to FIG. 2, isfirstly switched over to a lower frequency, resulting in a loweroperating voltage U, and then, at time-point t₃, is blocked or switchedoff. The switching transistor 24, therefore, also acts as a chopper asin the case of FIG. 1, the switching frequency of which isfrequency-modulated by the pulsed output signal, which acts as aswitching signal, of the oscillator 40 as a function of the fault signalE. The lower switching frequency of the chopper and of the output pulsesthereof leads to an increase in the reactance of the capacitor 26 and toa decrease in the reactance of the primary winding 27 and in its voltagedrop and, as a result, also in a decrease in the operating voltage U andthe current I. It is, however, also possible so to construct theoscillator 40 that the pulsed switching signal it generates ispulse-length-modulated as a function of the fault signal E, namely, insuch a manner that, at time-point t₂, the length of the pulses of theswitching signal is diminished and finally, at time-point t₃, is reducedto zero.

Additionally, it should be mentioned that the frequency of theoscillators 37 and 40, including the lower value of the frequency of theoscillator 40, is very much higher than the pulse frequency of the pulsegenerator 10.

The arrangement according to FIG. 4 differs from that according to FIG.3 essentially only in that the switching signals of the oscillator 40are fed to one input of one AND gate 41 for each switching transistor 9,and to the other input of the AND gates 41 there are fed control pulsesP₁ to P₆ from the relevant output of the control pulse generator 10. Theoutputs of the AND gates 41 are each connected to the control connectionof one switching transistor 24 for each switching transistor 9. Thehigh-frequency switching signal of the oscillator 40 acts, especially,as a carrier signal for the relevant, low-frequency control pulses P₁ toP₆. On the secondary side of each transformer 28, the carrier signal,having been amplitude-modulated by the relevant control pulses P₁ to P₆in the relevant AND gate 41, is demodulated by the rectification andsmoothing carried out by the diode 30 and the capacitor 31. In thatprocess, the carrier signal is suppressed so that the waveform of theoperating voltage largely corresponds to that of the control pulses. Theoperating voltage U is, at the same time, supplied to the controlconnection (not shown in FIG. 4) of the relevant driver stage 12, whichis still so constructed that it feeds the switching transistor 9downstream with control pulses (firing pulses) corresponding to theoperating voltage pulses and control pulses fed to it. In that process,the operating voltage U of the relevant driver stage 12 is, as afunction of the fault signal 3, reduced in stages with the aid of theoscillator 40 by means of frequency modulation or pulse-lengthmodulation and finally switched off, and the relevant driver stage 12 isswitched alternately on and off by means of the operating voltage pulsesand the relevant control pulses P₁ to P₆ before complete switching-off.The components 12, 24, 26 to 31 and 41 are provided separately for eachsemiconductor switch 9 in order to transmit galvanically separated notonly the oscillator pulses but also the control pulses P₁ to P₆ to thehigh-voltage-operated switching device 7 with the result that the otherswitching circuits which control the primary side of the galvanicisolation stages, here the transformers 28, can be operated using lowvoltage and yet no additional galvanic isolation stages are required forthe transmission of the control pulses P₁ to P₆.

Modifications of the illustrated embodiments may, for example, consistin providing, for the purpose of galvanic isolation, not the transformeror the transformers 28 but rather other galvanic isolation stages, forexample opto-couplers. Furthermore, a two-way rectifier can be providedon the secondary side of the transformer or the transformers 28. It isalso possible, in the embodiment according to FIG. 3, to omit thevoltage controller 32, the diode 19, the capacitor 20, the switchingtransistor 24 acting as chopper, the capacitor 26 and the transformer 28and to connect the diode 30 and the capacitor 31 directly to thesecondary winding 16 of the transformer 14 and then to control theswitching transistor 15 directly by means of the oscillator 40 as afunction of the fault signal E, should no galvanic isolation between thehigh-voltage and low-voltage sides be necessary or desired. Instead ofhaving only one switching transistor 24 for each chopper, the chopper(s)according to FIG. 3 and FIG. 4 can also be provided withpush-pull-operating switching transistors, such as the switchingtransistors 24, 25, 24′, 25′ according to FIG. 1. A chopper havingpush-pull operation (having the switching transistors 24-25′ accordingto FIG. 1) has the advantage that the primary winding 27 of thetransformer or transformers 28 is operated with alternating current and,as a result, the ripple and, therefore, the extent of smoothing requiredon the secondary side of the transformer(s) is reduced. Finally, theinvention can be used not only in a semiconductor switching device 7having several semiconductor switches 9, such as an inverter, but alsoin a semiconductor switching device having only one semiconductor switch9.

What is claimed is:
 1. Circuit for blocking a semiconductor switchingdevice on overcurrent, the semiconductor switching device having atleast one continuously driven semiconductor switch, which circuitcomprises a driver circuit having a driver stage for each semiconductorswitch, a control pulse generator for producing control pulses, whichare fed in operation to a control input of the semiconductor switchingdevice via the driver circuit, and a monitoring device which measurescurrent flowing through the semiconductor switching device and which,when an overcurrent occurs, generates a fault signal, which initiatesblocking of the semiconductor switching device, the driver circuithaving means to switch its operating voltage over briefly upongeneration of the fault signal to a lower interim value corresponding toa lower current through the semiconductor switching device and then,within a maximum permissible duration for loading the semiconductorswitching device with an overcurrent, to switch off the operatingvoltage completely.
 2. Circuit according to claim 1, including, forseveral of said semiconductor switches jointly supplied from oneoperating voltage source, said monitoring device in a supply line commonto all the semiconductor switches for measuring the current flowingthrough the semiconductor switches.
 3. Circuit according to claim 1, inwhich driver stages for the semiconductor switches are all supplied froma common operating voltage source which is galvanically isolated fromthe driver stages and which, as a function of the fault signal, isarranged to be switched over to the interim value and then switched off.4. Circuit according to claim 3, in which the operating voltage sourceof the driver circuit is a direct-current voltage source having achopper controlled by a pulsed switching signal and a transformer havinga secondary winding for each driver stage and a rectifying circuitconnected to the secondary winding, each driver stage being connected tosaid at least one semiconductor switch, and the switching signal thatcontrols the chopper being frequency- or pulse-length-modulated as afunction of the fault signal.
 5. Circuit according to claim 4, in whichthe control pulses of the control pulse generator are combined with theswitching signal controlling the chopper by means of an AND gate. 6.Circuit according to claim 3, in which the control pulses and theoperating voltage for each driver stage are transmitted by means of ahigh-frequency carrier signal of an oscillator that is common to all thedriver stages, by a galvanic isolation stage.
 7. Circuit according toclaim 1, in which the control pulses of the control pulse generator arefed in operation to a control input of the driver circuit.
 8. Circuitaccording to claim 1, in which the operating voltage source of thedriver circuit has an output for a high, normal value and an output forthe low, interim value, one of which outputs can be selected for thesupply of the operating voltage as a function of the fault signal. 9.Circuit according to claim 8, in which one output is connected by adiode to one end of a switching path of a controllable switch, the otheroutput is connected by a diode to another end of the switching path andto the driver circuit, and the switch is arranged to be switched as afunction of the fault signal.